Research On Electromagnetic Docking Device Topology And Control Strategy

With the development of Space technology,spacecrafts are becoming miniaturized,modularized,and distributed,and are widely used to construct mega-constellations.These spacecrafts may need to perform multiple undocking and docking processes during their missions.Traditional thruster-based docking has several obvious disadvantages such as propellant consumption,plume contamination,and strong impacts,which pose significant risks in those missions requiring repeated dockings.Thus,the electromagnetic docking,which is internal force powered,renewable,clean,and highly controlled with precision,has become the potential alternative.In most of the existing research on electromagnetic docking technology,only electromagnetic force is utilized for relative distance control,while the coupling electromagnetic torque is considered as disturbance and counteracted using other actuators such as reaction flywheels or thrusters.There have been no comprehensive studies related to docking using both and only electromagnetic forces and torques.This paper addresses this issue and the main contents of this paper include the following parts:(1)A mechanically guided electromagnetic docking device is proposed,which does not require sensors to detect the relative motion of the two satellites,and uses only electromagnets as actuators but incorporates a docking port mechanism to limit excessive misalignments and aid in guidance.During docking the relative motion of the two satellites is in 6-Degree-OfFreedom(6DOF).A specially designed iron-core electromagnet is proposed that significantly increses the the electromagnetic force within the docking range compared to a coreless coil.In order to compensate for the limited attitude correction ability of the electromagnet,a docking port is designed as a taper hole-taper rod coordination mechanism to restrict the non-docking axial movement so that the docking is carried out under the electromechanical cooperation.Based on the circuit model of the electromagnetic docking device,an algorithm for calculating the distance between two satellites on the basis of the high-frequency injection(HFI)method has been developed.In the specific implementation,first,a high-frequency(HF)voltage is injected into one of the two electromagnets;second,the HF currents induced by both electromagnets are measured and their respective root-mean-squares(RMSs)are calculated;third,two RMSs are substituted into a specific formula to obtain a variable carrying distance information;finally,the variable is utilized to calculate the distance estimation using the look-up table interpolation method.This paper presents a closed-loop electromagnetic docking controller which includes an outer distance loop and an inner speed loop and adopts the distance estimation as the feedback.The proposed sensorless electromagnetic docking method is verified by the distance estimation tracking response test and the ground-based docking test.The results indicate that low-impact docking can be achieved under the initial condition that the two satellites have a certain degree of misalignment.(2)A novel electromagnetic docking device topology,which consists of only electromagnetic coils,is proposed for realizing 6DOF soft docking between two satellites in Space.The target satellite has a main coil and four secondary coils located inside this main coil evenly arranged around the axis;the chaser satellite possesses a main coil accompanied by six and four evenly arranged secondary coils inside and outside the main coil,respectively.The control of 6DOF relative motion between two satellites is reorganized into 4 parts: relative distance control,relative translation control,relative attitude control and relative roll control.The control of relative translation and relative attitude are designed to always be be carried out simultaneously,hence they are combined as relative pose control.The entire electromagnetic docking process is divided into four stages: pulling the two satellites closer,relative pose adjustment,relative roll,and final docking.In the pulling stage,relative distance control is performed to bring the two satellites closer to an appropriate distance in order to improve the efficiency of the subsequent steps;in the relative pose adjustment stage,relative pose control and relative distance control are excuted to align the two satellites while maintaining the distance between them;in the relative roll stage,relative roll control is performed to make the two satellites to be rolled relative to each other to a proper angle in preparation for the locking after the entire docking process;in the final docking stage,relative distance control is excuted again to pull the two satellites to the final docking point.(3)The relative distance control of the 6DOF electromagnetic docking device is performed by the two main coils.In this paper,an engineering-friendly relative distance closedloop control system is designed,and several dynamics simulations are performed to validate the proposed control strategy and to explain some relevant details of the electromagnetic docking process planning.(4)The relative pose control of the 6DOF electromagnetic docking device is performed by the main coil of the target and the internal and external secondary coils of the chaser.During operation,the main coil of the target satellite is energized with DC current,while the currents in the internal/external secondary coils of the chaser satellite are actively controlled.To remove the coupling between the pitch/yaw torque and translational force,the internal and external secondary coils of the chaser satellite interact with the main coil of the target satellite to perform the control of relative pitch/yaw and relative translation,respectively,so relative pose control can be achieved.The torque and force vectors exerted by the secondary coils of the chaser satellite are synthesized onto the pitch and yaw axes of the body frame.According to their spatial composition relationship,the assignment formulas are presented for calculating the magnetic moment vectors of each secondary coil of the chaser,and the equations that convert the pitch/yaw torque and translational force setpoints to the setpoints of the coil currents are given.The feasibility of the proposed decoupling method is mathematically proved.The controllers regulating pitch/yaw,translation,and distance utilize a three-loop cascaded structure that consist of an outer position loop,a middle velocity loop and an inner current loop,and the stability of the entire relative pose control system is analyzed.The control strategy is verified by dynamics simulation.(5)The relative roll control of the 6DOF electromagnetic docking device is performed by the internal secondary coils of the two satellites.During operation,the internal secondary coils of the target satellite are energized with DC current,while the currents in the internal secondary coils of the chaser satellite are actively controlled.The electromagnetic force/torque model is built by utilizing the frequently-used far field model.Based on the fundamental components extracted from that model,this paper proposes a real-time magnetic moment vector distribution formula that simply generates a constant roll torque.It is pointed out that a “dead point” will appear if two satellites utilize the same coil arrays,and a method to solve the “dead point” is proposed.The equation that converts the roll torque setpoint to the setpoints of the coil currents is presented,and a three-loop cascaded structure positioning controller composed of angle,angular velocity,and current loop is developed.The proposed electromagnetic force/torque model is verified by finite element simulation,and the control strategy is validated by dynamics simulation and ground-based tests.